Tackle the remaining declensions — the large and varied third, plus the smaller fourth and fifth. The third declension is the biggest and most diverse, but once you know the patterns, you'll recognize them everywhere.
Estimated Time: 45–55 minutes
The third declension is the largest and most varied. It includes all three genders, and nouns can end in almost anything in the nominative. The genitive singular ending -is is the key identifier.
Model: rēx, rēgis (m.) — king
| Case | Singular | Plural |
|---|---|---|
| Nominative | rēx | rēgēs |
| Genitive | rēgis | rēgum |
| Dative | rēgī | rēgibus |
| Accusative | rēgem | rēgēs |
| Ablative | rēge | rēgibus |
| Vocative | rēx | rēgēs |
The nominative singular is often irregular — the stem appears in the genitive. Drop the -is from the genitive to find it: rēg-is → stem rēg-. mīlit-is → stem mīlit-. corpor-is → stem corpor-. Always use the genitive stem for all other cases.
Model: corpus, corporis (n.) — body
| Case | Singular | Plural |
|---|---|---|
| Nominative | corpus | corpora |
| Genitive | corporis | corporum |
| Dative | corporī | corporibus |
| Accusative | corpus | corpora |
| Ablative | corpore | corporibus |
Just like second declension: nominative = accusative, and the plural nom/acc ends in -a. This holds for neuter nouns in every single declension.
Some third declension nouns have slightly different endings — called i-stems. They're identified by patterns in their nominative/genitive:
| Type | Rule | Examples |
|---|---|---|
| Parisyllabic | Same number of syllables in nom. & gen. | cīvis, cīvis (citizen); nūbēs, nūbis (cloud) |
| Double consonant stem | Stem ends in two consonants | urbs, urbis (city); mōns, montis (mountain) |
| Neuter -e, -al, -ar | Nominative ends in -e, -al, -ar | mare, maris (sea); animal, animālis (animal) |
i-stem differences: genitive plural -ium (not -um), neuter nom/acc plural -ia (not -a), ablative singular sometimes -ī (neuters).
| Latin | English | Latin | English | |
|---|---|---|---|---|
| rēx, rēgis (m.) | king | lēx, lēgis (f.) | law | |
| mīles, mīlitis (m.) | soldier | vōx, vōcis (f.) | voice | |
| homō, hominis (m.) | person, human | pāx, pācis (f.) | peace | |
| dux, ducis (m.) | leader | lūx, lūcis (f.) | light | |
| pater, patris (m.) | father | māter, mātris (f.) | mother | |
| corpus, corporis (n.) | body | tempus, temporis (n.) | time | |
| caput, capitis (n.) | head | nōmen, nōminis (n.) | name |
Model: exercitus, exercitūs (m.) — army. Identified by genitive singular -ūs.
| Case | Singular | Plural |
|---|---|---|
| Nominative | exercitus | exercitūs |
| Genitive | exercitūs | exercituum |
| Dative | exercituī | exercitibus |
| Accusative | exercitum | exercitūs |
| Ablative | exercitū | exercitibus |
Common 4th declension: senātus (senate), manus (hand — feminine!), cornū (horn — neuter!), domus (house — irregular).
Model: rēs, reī (f.) — thing, matter, affair. The smallest declension. Identified by genitive -ēī.
| Case | Singular | Plural |
|---|---|---|
| Nominative | rēs | rēs |
| Genitive | reī | rērum |
| Dative | reī | rēbus |
| Accusative | rem | rēs |
| Ablative | rē | rēbus |
Key 5th declension words: rēs (thing — extremely common), diēs (day — masculine!), fidēs (faith), spēs (hope), rēs pūblica (republic, lit. "the public thing").
1. How do you find the stem of a third declension noun?
2. What makes a noun an "i-stem"?
3. Which fifth declension noun is famously masculine (unlike most in its declension)?
✦ Third declension is the largest — all genders, genitive -is. Find the stem from the genitive.
✦ i-stems (parisyllabic, double-consonant, neuter -e/-al/-ar) have gen. pl. -ium and neuter pl. -ia.
✦ Fourth declension: gen. -ūs, mostly masculine. Key: exercitus, senātus, manus (f!), cornū (n!).
✦ Fifth declension: gen. -ēī, mostly feminine. Key: rēs (thing), diēs (day — m!), rēs pūblica.
✦ Neuter rule ALWAYS holds: nom = acc, plural nom/acc in -a (or -ia for i-stems).